1. Field of the Invention
The present invention relates to filters for the removal of heavy metals, processes for preparing such filters and the use of such filters for removing heavy metals from aqueous solutions contaminated therewith.
2. Prior Art
The broad applicability of ion exchange chromatography to separate inorganic ions, has made it a powerful and versatile tool for chemical separations. The technique originally used non-synthetic or natural compositions as ion exchange media, for example, cellulose, clay and other minerals which contained mobile ions that could be exchanged with ionic materials in the surrounding solute phase. Due to the low ion exchange capacity of such natural compositions which limited their use, synthetic organic ion-exchange polymers were developed.
Ion exchange resins were among the first generation of synthetic ion exchange materials. The fundamental structure of ion exchange resins is an elastic three-dimensional hydrocarbon network comprising ionizable groups, either cationic or anionic, chemically bonded to the backbone of a hydrocarbon framework. The network is normally fixed, insoluble in common solvents and chemically inert. The ionizable functional groups attached to the matrix carry active ions which can react with or can be replaced by ions in the solute phase. Therefore, the ions in the solute phase can be easily exchanged for the ions initially bound to the polymeric resins. Typical examples of commercially available ion exchange resins are the polystyrenes cross-linked with DVB (divinylbenzene), and the acrylates or methacrylates copolymerized with DVB. In the case of polystyrene, a three-dimensional network is formed first, and the functional groups are then introduced into the benzene rings through chloromethylation.
Cation ion exchangers have fixed anionic functional groups, e.g., --CO.sub.2 --M+, --SO.sub.3 --M+, --PO.sub.3 .dbd. H.sub.2 +. Anion ion exchangers have fixed cationic functional groups, e.g., --NH.sub.2, --NRH, --NR.sub.2 -- N+R3X--.
Commercial ion-exchange resins are designed to have high practical capacities for their specific applications. Several major factors are considered in choosing an appropriate ion-exchange resin, e.g., selectivity, porosity, resin particle size, and flow rate.
After choosing an ion-exchange media with a high selectivity for a specific ion required to be exchanged, the next factor to consider is efficiency. Ion-exchange efficiency is determined by porosity and particle size. Porosity is controlled by the degree of swelling of the polymer matrix, which, in turn, is determined by the density of crosslinking. This swelling network permits the diffusion of ions in and out of the ion-exchange matrix. However, if the ions to be exchanged are large, the reaction can only take place on the media. To overcome this problem, ion exchange media with large fixed pores and/or large contact surfaces have been developed, e.g., rigid macroreticular ion-exchange materials, so that flocculants can be easily removed from or passed through the large pores. However, even with these improvements, such ion-exchange media still have disadvantages, for example, limited flow rates (&lt;100 bed volume/hour), and channeling problems. When a high flow rate (&gt;&gt;100 bed volume/hour) is applied to an ion exchange bed comprising such media, the efficiency of the ion-exchange reaction is decreased due to shorter contact times resulting in incomplete reactions. The pressure drop is also increased across the bed.
Normally, coarse, rigid ion-exchange resins are used for high flow conditions to reduce problems associated with the high pressure drops experienced. Such ion exchange materials, however, show low efficiencies even for macroporous or macroreticular type ion exchange materials. Finer mesh ion-exchange materials give higher efficiencies, but cause higher operating pressures and high head losses during back washing. It is clear that the kinetics of the ion-exchange reaction depend on the porosity of the polymer matrix and the total contact surface area of the liquid-polymer interface. Thus, where high flow conditions apply, the macroporosity and the particle size become the most important factors.
As with every unit process, ion-exchange processes have their limitations and problems. However, new technologies can be developed to minimize these limitations and problems. These limitations are due to many factors, for example:
1. The nature of the ion-exchange material. PA0 2. The ions involved. PA0 3. The operating conditions. PA0 4. The quality of effluent required. PA0 5. The cost of the process. PA0 6. The fouling and interfering species.
In heavy metal removal, it is desirable to remove one or several metal cations from solutions containing concentrations of other similar metal cations. Selectivity differences exist between ion-exchange materials. For example, the relative selectivity of carboxylic acid-sodium salt ligands and metal cations at pH 5 is: EQU Hg&gt;&gt;Pb&gt;Cu&gt;Cd&gt;Zn, .about.Ni&gt;Ca&gt;Mg&gt;&gt;K, .about.Na
The relative selectivity of the ion-chelating group such as iminodiacetic acid linked to epoxy groups of poly (glycidyl acrylate) or poly (glycidyl methacrylate) coupled to a cellulose matrix at pH 4 is: EQU Cu&gt;Pb&gt;&gt;Ni&gt;Zn&gt;Cd&gt;Co&gt;Ca&gt;Mg&gt;K, .about.Na
The relative selectivity of similarly immobilized ethylene diamine tetraacetic acid at pH 4 is: EQU Cu&gt;Pb&gt;Co&gt;Zn, .about.Ni, .about.Ca, .about.Mg&gt;K, .about.Na
Thus, as can be seen, ion-exchange media with carboxylic acid, iminodiacetic acid, and ethylene diamine tetraacetic acid-sodium or potassium salts can remove heavy metals such as Pb, Cu, Cd and Zn, very efficiently from water containing calcium, magnesium, potassium and sodium ions. At a pH below 3, however, ionization of carboxylic acid is depressed and capacities are reduced.
At a pH higher than 8, heavy metal hydroxides precipitate and form colloids, with particle sizes of from about 0.05 to several hundred microns, depending on ion concentration and other water characteristics. When the pH is higher than 9, soluble anionic complexes or insoluble anionic colloids are formed, which depend on concentrations of metal ions, anionic ligands and the characteristics of the water. Both metal hydroxide colloids and anionic species cannot be exchanged with cation exchangers. Colloids are usually removed through mechanical filtration or absorption methods. Anionic colloids and complexes are usually removed through mechanical filtration and anion exchangers.
In an ammoniacal stream (pH=9) containing, for example, ammonium sulfate heavy metals such as Co, Ni, Cd, Cu and Zn form cationic complexes, which can be removed by chelation ion-exchangers.
The relative selectivity between cations of a sulfonated copolymer of styrene and divinylbenzene are close. Taking lithium as the base, the relative selectivity coefficients are as follows:
______________________________________ Counter Ion Relative Selectivity Coefficient ______________________________________ Li+ 1.00 H+ 1.27 Na+ 1.98 Mn.sup.2 + 2.75 K+ 2.90 Mg.sup.2 + 3.29 Zn.sup.2 + 3.47 Cu.sup.2 + 3.85 Cd.sup.2 + 3.88 Ca.sup.2 + 5.16 Ag+ 8.51 Pb.sup.2 + 9.91 ______________________________________
It is clear that this type of ion-exchange material cannot effectively remove trace heavy metal ions from water containing high concentrations of calcium and magnesium. Such an ion-exchange material only works well in the absence of or in the presence of only minor amounts of these interfering ions. Such ion-exchange media are primarily useful only at pH 2 and above. However, such material still cannot remove colloids and anionic species. The above cation exchange resins are normally regenerated with strong acid (5-10%). However, in the case of weak acid ion exchange resin (carboxylates), chelating regenerants, such as sodium citrate and ethylenediamine tetracetic acid (Na-salts) may be used to remove metal ions, the ion exchange resins being actually regenerated under basic conditions.
Weak bases such as --NH.sub.2, --NRH and --NR.sub.2, and strong bases such as --N+R.sub.3 X-- can remove anionic species from water, such as C1--, SO.sub.4 .dbd., NO.sub.3 --, anionic colloidal particles, anionic complexes and anionic organic compounds. Thus, such ion exchange materials can be used for removal of anionic heavy metal complexes and colloids. Thus, in order to remove heavy metals from water, conditions such as pH and forms of heavy metal should be determined before choosing the type of ion-exchange media needed.
The foregoing review of the prior art, its advantages and drawbacks, leads to the conclusion that there exists a need for an ion exchange chromatography-based purification device which will have high stability, high porosity, high flow rate, be relatively incompressible and control gelation, in conjunction with high filtration efficiency for heavy metal removal at low and high pH. It is the industrial level of manufacturing, especially where the aforementioned drawbacks have had their most important effect and where this need is the strongest.
Industrial scale separation materials comprising fibrous matrices with particulate immobilized therein are described in U.S. Pat. No. 4,384,957 to Crowder, III, et al (1983), which is incorporated herein by reference. Crowder describes a composite fiber material formed by wet laying a sheet from an aqueous slurry of ion-exchange resins or ion-exchange fibers, particulate, small refined fiber pulp and long soft fiber pulp. The purpose of the soft long fiber is to physically hold the clumps of particulate material and refined pulp together. Sheets are formed from a wet slurry by vacuum filtration, wherein the long fibers form in a plane which is perpendicular to the direction of flow of the chromatographic carrier fluid. This permits channels to form in the composite material which are perpendicular to the direction of flow, permitting these materials to serve as internal flow distributors. The resulting matrix structure has proven to be an effective way of eliminating channeling defects through internal flow distribution mechanisms.
U.S. Pat. No. 4,663,163 to Hou et al describes a modified polysaccharide material and U.S. Pat. No. 4,724,207 to Hou et al describes a modified silica material. Each of these materials are preferably modified by a synthetic polymer covalently bonded to the material. Preferably the synthetic polymer is a copolymer made from a free radical polymerization of:
(a) a polymerizable compound containing an epoxy group capable of direct covalent coupling to a hydroxy group of said polysaccharide and a vinyl group, capable of free-radical polymerization; and
(b) a polymerizable compound having the formula ##STR1## wherein R is an alpha, beta-unsaturated polymerizable radical, R' and R" are the same or different C.sub.1 -C.sub.6 alkyl or alkanoyl groups, and R'" is a direct bond or a C.sub.2 -C.sub.3 alkyl group, wherein R' and R" taken together with the N atom may form a heterocyclic ring.
The polymerizable compound (b) may also have the following formula: ##STR2## wherein
R.sub.4 is hydrogen or methyl;
A is CO, or SO.sub.2 ;
X is OH, OM (where M is a metal ion), OR.sub.5 (where R.sub.5 is a straight or branched chain C.sub.1 -C.sub.18 alkyl group), OR.sub.6 OH (where R.sub.6 is a straight or branched chain C.sub.2 -C.sub.6, alkyl or aromatic group), NR.sub.7 R.sub.8 or NR.sub.7 R.sub.8 R.sub.9 (where R.sub.7 is the same or different as R.sub.8 which is the same or different as R.sub.9, and are hydrogen, R.sub.5 or R.sub.6 OH);
AX may also have formula (5): ##STR3## wherein Y is --CO.sub.2 --, --CH.sub.2 CO.sub.2, --SO.sub.3 --, CH.sub.2 SO.sub.3, --PO.sub.4 H--, CH.sub.2 PO.sub.4 H--, --CH.sub.2 N(CH.sub.2 COO--).sub.2, --CH.sub.2 --NR.sub.7 R.sub.8, or --CH.sub.2 --NR.sub.7 R.sub.8 R.sub.9, or the corresponding free acid, ester, or partial ester groups, as described previously.
In these formulas, the groups R.sub.7, R.sub.8 ; R.sub.7, R.sub.9 ; or R.sub.8, R.sub.9 may form a 5-7 membered heterocyclic ring with the nitrogen atom. R.sub.7, R.sub.8, and R.sub.9 are as previously defined.
Hou et al, does not teach or suggest that such materials are useful for removing heavy metals from aqueous solutions contaminated therewith and does not teach or suggest the media of this invention.